CHARACTERIZATION OF MICROSTRUCTURE IN STEELS BY MAGNETIC TECHNIQUES -AN OVERVIEW

Magnetic techniques are highly sensitive for microstructural character isations in ferromagnetic materials like steels and can be used in the areas of quality control and remaining life assessments. Tile basis o f these techniques are related to the domain nucleations and domain dy namics with respect to the various types of microstructural features s uch as grain boundaries. second phase precipitates, residual stresses, dislocations etc. The microstructural features act as sites for magne tic domain nucleation and pinning centres against magnetic domain wall motions -both the phenomena being responsible for the magnetic behavi our of a ferromagnetic material like steel. Different types of domain walls (180 degrees and 90 degrees walls) interact differently with the microstructural features. There are two broad classes of magnetic tec hniques, namely analysis of magnetic hysteresis loop parameters and th e analysis of magnetic and acoustic Barkhausen noise signal. The hyste resis loop parameters are coercivity. retentivity, permeability, diffe rential permeability etc. The reasons for a generally observed lack of straight forward relationships is due to complex domain patterns and their complex interactions with the microstructural features. The pape r discusses the principles based on which various types of magnetic pa rameters call be used in specific application cases. Examples are give n to show the relevance of the various techniques.